The moment Microsoft announced it would lock other browsers out of being installed on Windows RT, we all knew regulatory bodies the world over were wringing their hands. Today, this has been confirmed: in the wake of an investigation into Microsoft not complying with the existing antitrust rulings regarding browser choice, the EU has also announced it's investigating Windows 8 x86 and Windows 8 RT (ARM).

Antitrust matters aren't about killing monopolies in existing categories, it's about making sure those monopolies aren't used to extend it into new markets.

You're erecting a strawman. Nothing I said contradicts that point.

What Microsoft is doing with Windows 8 is text book antitrust behaviour: They use their x86 monopoly to push Metro, to reap the rewards (mindshare, applications) on ARM devices and in the mobile space.

Bullshit. Having x86 monopoly power doesn't give Microsoft any leverage to extend its dominance into the ARM market. Zero. Zilch. Nada. Microsoft can't strong-arm OEMs to tie x86 and ARM together. It can't force consumers to buy ARM machines. It's an entirely separate market and category. Go back and read the market definition for the antitrust trial. It's specifically for "x86-based operating systems". Not ARM operating systems. Furthermore, apps written for x86 won't run natively on ARM. The apps need to be ported/recompiled for ARM.

I'm sure that you'll come back with some nebulous statement about how the common Metro interface, itself, somehow magically transforms into market power on ARM. But that's complete bullshit. The interface gives Microsoft no more power than, say, Google's standardization on the Java runtime on Android gives it power over the ARM market.